PIN photodetector

ABSTRACT

A PIN photodetector includes an n-type semiconductor layer, an n-type semiconductor cap layer, a first plurality of p-type regions located within the n-type semiconductor cap layer and separated from one another by a distance d1, and an absorber layer located between the n-type semiconductor layer and the n-type semiconductor cap layer including the first plurality of p-type regions. The plurality of p-type regions are electrically connected to one another to provide an electrical response to light incident to the PIN photodetector.

TECHNICAL FIELD

The invention relates generally to photodetectors, and in particular toPIN-type photodetectors utilized in optical detection systems.

BACKGROUND

Photodetectors operate to convert incident light into an electricalresponse. A commonly utilized type of photodetector is the PIN diode orphotodetector, which comprises an undoped or lightly doped intrinsicsemiconductor region located between p-type and n-type semiconductorregions (hence the name P-I-N photodiode). To operate as aphotodetector, the PIN diode is reverse biased to create a depletionregion located largely within the intrinsic region of the PIN diode.Under reverse bias, the PIN diode does not conduct except for anundesirable “dark current” that results from the spontaneous creation ofelectron-hole pairs. Light incident to the intrinsic region of the PINdiode creates an electron-hole pair within the intrinsic region. Thereverse bias field of the depleted region sweeps the carriers out of theregion via a mechanism known as “drift”, or a process of “diffusion” inthe undepleted region, wherein the collection of the charge carrierscreates an electrical response (e.g., current) detectable by downstreamsignal processing devices.

Typical designs of PIN photodetectors for use in optical detectionsystems focus on the efficiency with which the photodetector convertsincident light to a detectable electrical response, reducing thecapacitance of the PIN photodetector, reducing the series resistance ofthe PIN photodetector as seen by downstream components, increasing theresponse time and reducing noise (e.g., leakage/dark current) generatedby the PIN photodetector. Some of these design goals are in conflictwith one another. For example, a reduction in capacitance is typicallyachieved by increasing the thickness of the intrinsic region. However,an increase in the thickness of the intrinsic region results in anincrease in noise as a result of increased bulk intrinsic material thatleads to an increase in dark current generated by the spontaneousgeneration of electron-hole pairs within the intrinsic region.

It would therefore be beneficial to provide a PIN photodetector that iscapable of satisfying/improving the performance of PIN photodetectors.

SUMMARY

According to some aspects, the present disclosure describes a PINphotodetector that includes an n-type semiconductor layer, an n-typesemiconductor cap layer, a first plurality of p-type region locatedwithin the n-type semiconductor cap layer and separated from one anotherby a distance d₁, and an absorber layer located between the n-typesemiconductor layer and the n-type semiconductor cap layer including thefirst plurality of p-type regions. The plurality of p-type regions areelectrically connected to one another to provide an electrical responseto light incident to the pin photodetector.

According to some aspects, an optical detection system includes a PINphotodetector and a signal conditioning circuit connected to receive theelectrical response generated by the PIN photodetector. In some aspects,the PIN photodetector includes an n-type semiconductor layer, a n-typesemiconductor cap layer, a plurality of p-type diffusion regionsdiffused within the n-type semiconductor cap layer and separated fromone another by a distance d₁, an absorber layer located between then-type semiconductor layer and the n-type semiconductor cap layer. Theplurality of p-type diffusion regions generate a plurality of depletionregions within the absorber layer, wherein each of the depletion regionsare separated from adjacent depletion regions by a distance d₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams of a light detection system utilizinga PIN photodetector according to some embodiments.

FIG. 2 is a cross-sectional view of a PIN photodetector according tosome embodiments.

FIG. 3 is a top view of a PIN photodetector according to someembodiments.

FIG. 4 is a cross-sectional view of a PIN photodetector according tosome embodiments.

FIG. 5 is a top view of a PIN photodetector according to someembodiments.

DETAILED DESCRIPTION

This disclosure is directed to a PIN photodetector that utilizes aplurality of p-type semiconductor regions separated from one another. Atypical PIN photodetector comprising a p-type semiconductor region andan n-type semiconductor region, separated by an intrinsic semiconductorregion (hence the name P-I-N). The typical PIN photodetector relies on ap-type semiconductor region that is approximately equal in size to theoptical detection area. This disclosure describes a PIN photodetectorthat includes a plurality of p-type semiconductor regions. In someembodiments the plurality of p-type semiconductor regions are separatedby a distance d. In some embodiments the plurality of p-typesemiconductor regions are separated from one another—as a plurality ofislands. In some embodiments the plurality of p-type semiconductors areconnected to one another (for example, as a plurality of p-type rowsextending from a base, wherein the rows are separated from one anotherby a distance). As described in more detail below, dividing the p-typesemiconductor region into a plurality of individual p-type semiconductorregions provides a number of advantages, including low capacitance, lowresistance, and low noise. In some embodiments, the distance between thep-type semiconductor regions is selected based on the bandwidth and/orrise time associated with downstream components of the optical detectionsystem.

FIGS. 1A and 1B are simple views of an optical detection systems 100 and100′, respectively, utilizing a PIN photodetector according to someembodiments. In the embodiment shown in FIG. 1A, optical detectionsystem 100 includes focusing lens 102, filter 104, photodetector 106,signal conditioning circuit 108, decision-making circuit 110, userdevice 112, and micro-processor 116. The embodiment shown in FIG. 1B issubstantially the same as the embodiment shown in FIG. 1A, wherein theoptical detection system 100′ includes focusing lens 102′, filter 104′,photodetector 106′, signal conditioning circuit 108′, decision-makingcircuit 110′, user device 112′, and micro-processor 116′. In addition,optical detection system 100′ includes a direct current (DC) blockingcapacitor at the input of signal conditioning circuit 108′.

In some applications the optical detection systems 100 and 100′ areutilized to detect a modulated light input. Modulated light sources mayinclude light sources generated by one or more of Super luminescentLight Emitting Diodes (SLEDs), light-emitting diodes (LEDs), laserdiodes, vertical-cavity surface-emitting laser (VCSELs), solid-statelasers, and fiber lasers. Modulated light detection system 100 may beutilized in a number of applications, ranging but not limited to mobilephones, gaming systems, autonomous and semi-autonomous ground and aerialvehicles, military laser warning and event detection systems. Dependingon the application the modulated light detection system 100 may beimplemented as part of a sensor utilized to initiate a response based onthe timing, position, shape, direction, or frequency content of themodulated light received. In other embodiments, optical detectionsystems 100 and 100′ may be utilized to detect non-modulated lightsources.

In the embodiment shown in FIG. 1, light including—both background lightand modulated light—is provided via a focusing lens 102 to filter 104,which acts to filter out or remove background light and/or other sourcesof unwanted light. The modulated light is provided to photodetector 106,which generates an electrical response (e.g., current pulse) in responseto the incident modulated light. In a number of applications,photodetector 106 is a large area detector capable of achieving largefields of view. As discussed in more detail with respect to FIGS. 2-4below, photodetector 106 may utilize a photodiode such as a PINphotodiode to generate the electrical response to the input modulatedlight. In general, it is desirable for the PIN photodiode to present alow capacitance, a low resistance, good/fast response time, and lownoise. For example, the capacitance and series resistance presented bythe PIN photodiode 106 define the “RC time constant” associated with thePIN photodiode 106. The RC time constant determines the speed at whichcurrent pulses can leave the photodetector 106 and flow into theamplifier. Additionally, the capacitance of the PIN photodiode combineswith the total resistance of the PIN photodiode and the signalconditioning circuit 108 to form an RC time constant associated with thelight detection system 100. It is therefore desirable to minimize thecapacitance and the resistance of the PIN photodiode to reduce the RCtime constant of the photodiode and of the light detection system.

Signal conditioning circuit 108 filters/amplifies the electricalresponse generated by the photodetector 106 to generate an electricalresponse that can be interpreted/utilized by decision-making circuit110. For example, in the embodiment shown in FIG. 1A, signalconditioning circuits 108 utilizes one or more transimpedance amplifier114 to filter and amplify the electrical signal received from thephotodetector. In some embodiments signal conditioning circuit 108 mayutilize one or more of passive filters, second and third stageamplification, active filters, level shifting, and/or additional signalconditioning/processing. In the embodiment shown in FIG. 1B, a directcurrent (DC) blocking capacitor C₁ (also referred to as an “alternatingcurrent (AC) coupling capacitor”) is provided at the input oftransimpedance amplifier 114 to block or filter DC signals (e.g., steadystate or DC bias signals), while allowing modulated or pulsed signals toflow into the transimpedance amplifier 114.

Various attributes of the photodetector 106 determine the level ofsignal conditioning required. As discussed above, these include one ormore of photodetector capacitance, resistance, response time (e.g.,bandwidth), and noise. In general, it is desirable for the photodetectorcapacitance and resistance to be low, for the response time to be asfast or faster than the bandwidth of the signal conditioning circuit 108and/or decision-making circuit 110. In some embodiments, the attributesassociated with the response speed (e.g., capacitance, resistance,transit time of carriers) are selected such that the response speed ofthe photodetector 106 is approximately equal to the bandwidth (e.g.,response speed) of downstream elements—such as the signal conditioningcircuit 108 and/or decision-making circuit 110. In some embodiments, theresponse speed of the photodetector 106 is selected to be approximatelyequal to the lowest bandwidth (e.g., slowest) component included in theprocessing stream that includes signal conditioning circuit 108 and/ordecision-making circuit 110. As described in more detail with respect toFIG. 2, below, the layout of the PIN photodetector 106 may be selectedto provide the desired attributes.

In some embodiments, decision-making circuit 110 is utilized to provideadditional processing/calculations with respect to the detectedmodulated light signal. In some embodiments, decision-making circuit 110includes a micro-processor 116 for analyzing the received signals. Inother embodiments, one or more analog circuit, digital circuits,application-specific integrated circuits (ASICs), microprocessors, etc.may be utilized to analyze/process the received signals. In someembodiments, decision-making circuit 110 provides an output to userdevice 112. For example, decision-making circuit 110 may makedeterminations regarding distance to a detected object based on receivedmodulated light signals, or may interpret data included in the modulatedlight signal. The result of the processing/calculations performed bydecision-making circuit 110 may then be provided to user device 112 fordisplay and/or action.

FIG. 2 is a cross-sectional view of a PIN photodetector 206 according tosome embodiments. The PIN photodetector 206 may be utilized inconjunction with optical detection systems, such as those described withrespect to FIGS. 1A and 1B above. PIN photodetector 206 may be utilizedas a back-illuminated photodetector or front-illuminated photodetector.In some embodiments, the PIN photodetector 206 includes cathode contact208, anti-reflective coating 210, n-type layer 212, absorber layer 214,n-type cap layer 216, surface passivation layer 218, anode contacts 220a, 220 b, and 220 b, guard ring contact 222, p-type diffusion layers 224a, 224 b, and 224 c, guard ring p-type diffusion 225, depleted regions226 a, 226 b, and 226 c, and guard ring depleted region 228.

In contrast with typical photodetectors utilizing a single, large p-typediffusion region and a resulting large depletion region for collectingcharge carriers, the embodiment shown in FIG. 2 utilizes a plurality ofp-type diffusion regions separated from one another by a distance. Asdiscussed above, the term “plurality of p-type diffusion regions”includes a plurality of p-type diffusion regions separated from oneanother—for example as a plurality of islands. The term “plurality ofp-type diffusion regions” also includes a plurality of p-type diffusionregions connected to one another—for example as a plurality of rowsextending like fingers from a p-type diffusion region wherein theplurality of rows are separated from one another by a distance. As aresult of the separation between the plurality of p-type diffusionregions, the depletion regions generated in response to the built-involtage and the bias voltage in the areas surrounding the p-typediffusion regions are also separated from one another by a distance,wherein in the region separating the depletion regions the absorberlayer is undepleted. Charge carriers created in response to incidentphotons in the undepleted regions are not subject to the built-inelectric field or reverse bias field, and therefore move according todiffusion mechanisms from the undepleted region to the depleted regionfor collection. This is in contrast with typical PIN photodetectors, inwhich the design objective is for the majority of the charge carrierscollected to have been created within the depletion regions andcollected via “drift” mechanisms. The distance between adjacent p-typediffusions 224 a, 224 b, and 224 c is selected such that charge carrierscreated in the undepleted regions of the absorber layer 214 arecollected by one of the depleted regions within the time constraints ofthe larger overall system (e.g., based on the bandwidth and/or rise timeof components associated with the signal conditioning circuit 108 anddecision-making circuit 110 shown in FIG. 1). In addition, utilizing aplurality of p-type diffusion areas—instead of a single large diffusionarea—decreases the area of the corresponding depletion regions 226 a,226 b, and 226 c as compared to a single large diffusion area andprovides a corresponding decrease in the parallel plate capacitanceassociated with the PIN photodetector 206. In some embodiments, thisallows the thickness of the absorber layer 214 to be decreased, therebydecreasing the “dark current” generated within the bulk of the absorberlayer 214.

In some embodiments, the photodetector makes use of III-V typesemiconductors, such as Indium-Phosphide (InP), Indium-Gallium-Arsenide(InGaAs), as well as other well-known III-V type semiconductormaterials. For example, in some embodiments the n-type layer 212 is anInP layer, the absorber layer 214 is a lightly n-doped InGaAs layer, andthe n-type cap layer 216 is an InP layer, with p-type diffusions 224 a,224 b, and 224 c within the n-type cap layer 216. The absorber layer 214may also be referred to in some PIN examples as an intrinsic layer,although most photodetectors do not rely on a wholly intrinsic (i.e.,undoped) semiconductor layer. In general, the intrinsic layer is lightlydoped (e.g., lightly n-doped). Utilizing a lightly n-doped InGaAs layerfor the absorber layer 214 allows for the depletion region to be morereadily formed at a lower bias voltage.

In the embodiment shown in FIG. 2, the n-type layer 212 is separatedfrom the n-type cap layer 216 by the absorber layer 214—which may belightly n-doped as described above. Incident light is provided to theabsorber layer 214, wherein the incident light (e.g., photons) createselectron-hole pairs (e.g., charge carriers) within the absorber layer214. For those charge carriers that are created within one of thedepletion regions 226 a, 226 b, and/or 226 c, the built-in electricfield and applied bias electric field (provided by voltage source 202)collect the charge carriers and provide them to the anode contacts 220a, 220 b, and 220 c and/or cathode contact 208. These charge carrierscontribute to the electrical response generated in response to thedetected photon or incident light. The mechanism for sweeping the chargecarriers from the depletion region to the anode contacts 220 a, 220 b,and 220 c is referred to as “drift”. For those electron-hole pairscreated in areas adjacent to the depletion region 226 a, 226 b, and 226c (i.e., the undepleted regions), the electric field is not as strongand charge carriers diffuse for a period of time until they eitherrecombine with one another or happen into a depletion region 226 a, 226b, and 226 c (or guard ring depleted region 228) which sweeps them outof the device. In particular, charge carriers that diffuse to thedepletion region 226 a, 226 b, and 226 c are swept out via the anodecontacts 220 a, 220 b, and 220 c, respectively, for collection, andtherefore also contribute to the photo-generated current delivered tothe terminals of the photodiode 206. As shown in FIG. 2, the anodecontacts 220 a, 220 b, and 220 c are connected together to provide acombined output to the signal conditioning circuit connected to receivethe electrical response. The time it takes for charge carriers todiffuse to the depletion region for collection is referred to as the“transit time”. Those charge carriers that arrive after an extendedperiod of time may not contribute to the peak of the electrical responsegenerated in response to the incident light. Charge carriers thatrecombine before reaching one of the depletion regions 226 a, 226 b,and/or 226 c or charge carriers that diffuse to the guard ring depletionregion 228 do not contribute to the photo-generated current.

Typically, photodetectors utilize a single diffused p-type region with acorresponding depletion region in order to ensure that most chargecarriers created by incident photons are located within the depletedregion and are therefore quickly collected via electron driftmechanisms. In contrast, PIN photodetector 206 employs two or morep-type diffusion regions (for example, p-type diffusion regions 224a-224 c shown in FIG. 2), each separated from one another by a distance.In some embodiments, the distance between adjacent p-type diffusionregions (e.g., between p-type diffusion region 224 a and p-typediffusion region 224 b) is represented by distance d₁, and each p-typediffusion region is separated from adjacent p-type diffusion regions bythe same distance d₁. As a result of the distance between adjacentp-type diffusion regions, the depleted regions 226 a, 226 b, 226 cassociated with each p-type diffusion region 224 a, 224 b, and 224 c,respectively, are separated by a distance. For example, in theembodiment shown in FIG. 2 the adjacent depletion regions 226 a, 226 b,and 226 c are separated by a distance d₃. In some embodiments, eachdepletion region is separated by the same distance, but this may bevaried in other embodiments. As a result of utilizing a plurality ofp-type diffusion areas 224 a, 224 b, and 224 c, the photodetector 206 ischaracterized by undepleted regions located between adjacent depletedregions (e.g., depleted regions 226 a, 226 b, and 226 c). Electron-holepairs generated within the undepleted regions by an incident photon mustdiffuse into one of the depletion regions 226 a, 226 b, and/or 226 c tobe collected. As discussed above, the time it takes for a charge carrierto diffuse to a depleted region for collection is known as the “transittime”. The transit time depends on one or more factors, such as thespeed at which carriers diffuse through the undepleted regions and thelength of the time the carriers will travel before recombining.Typically, the transit time for a given device is known.

In some embodiments, the distance d₃ between adjacent depletion regions226 a, 226 b, and c is selected such that the transit time of chargecarriers created with the undepleted regions satisfies the bandwidthrequirements of downstream signal processing components (e.g., operatingbandwidth). The reciprocal of bandwidth (e.g., 1/bandwidth) has units ofseconds, and can therefore be compared to the transit time of the chargecarriers. That is, the transit time of charge carriers must be less thanthe reciprocal of the bandwidth associated with one or more of thesystem components (e.g., signal conditioning circuit 108,decision-making circuit 110, etc.) to ensure that created chargecarriers have sufficient time to diffuse into the depletion region 226a, 226 b, or 226 c for collection. In some embodiments the transit timeof the charge carriers is selected to ensure that the charge carriersare collected in time to contribute to a useful signal response. In someembodiments, the threshold transit time is based on the rise time of theone or more downstream signal processing components, which may beapproximated by the equationt _(r)=0.35/Bandwidth(Hz)  Eq. 1where t_(r) is the rise time of the circuit. Having calculated thethreshold times for charge carriers to be collected—based either on thebandwidth or on the rise time of the downstream signal processingcomponents—the distance d₃ between adjacent depletion regions 226 a, 226b, and 226 c is determined such that a charge carrier located in theundepleted region (more particularly, a charge carrier locatedapproximately in the middle of the undepleted region) is capable ofdiffusing an adjacent depletion region 226 a, 226 b, or 226 c is lessthan or equal to the calculated threshold time. For example, if thesignal conditioning circuit 108 shown in FIG. 1 operates at a bandwidthof 10 MHz, then the rise time of the circuit may be approximated as0.035 μs. The distance d₃ between adjacent depletion regions 226 a, 226b, and 226 c is selected such that a charge carrier formed within theundepleted region (e.g., halfway between adjacent depletion regions)will diffuse to an adjacent depletion region in approximately 0.035 μsor less. For example, the selection criteria for the distance d₃ may beexpressed as follows:d ₃<2*(1/bandwidth)*(diffusion speed)  Eq. 2wherein d is the distance between adjacent depletion regions associatedwith the p-type diffusion regions, bandwidth is the operating bandwidthof downstream electrical components, and diffusion speed is the meanvelocity of charge carriers diffusing through undepleted regions. Ifutilizing the rise time, Eq. 2 could be re-written as:d ₃<2*(0.35/bandwidth)*(diffusion speed)  Eq. 3This ensures that the collected charge carriers contribute to the peaksignal provided to the downstream processing components (e.g., signalconditioning circuit, etc.). In this way, while the charge carriers arenot collected as fast as possible from the photodetector 206 becausethey rely on diffusion mechanisms rather than drift mechanisms, thecharge carriers are collected fast enough for the carriers to beincluded in the downstream signal.

In some embodiments, the distance d₃ between adjacent depletion regions226 a, 226 b, and 226 c is a function of both the distance betweenadjacent p-type diffusion regions 224 a, 224 b, 224 c and the reversebias voltage applied. The distance between p-type diffusion regions 224a, 224, and 224 c cannot be modified after fabrication. However, thereverse bias voltage may be modified during operation to selectivelyincrease/decrease the distance d₃ between depletion regions 226 a, 226b, and 226 c as required. In some embodiments, the distance d₅ betweenguard ring depletion region 228 and adjacent depletion regions (e.g.,depletion region 226 c) may be approximately equal to the distance d₃between adjacent depletion regions. In other embodiments, the distanced₅ may be greater than or less than the distance d₃.

As a result of the distance d₃ between the depletion regions 226 a, 226b, and 226 c, a large area of the PIN photodetector 206 is dominated bydiffusion mechanics, rather than drift mechanics. This is in contrastwith typical PIN photodetectors, which are designed with large depletionareas to maximize drift mechanics.

In some embodiments, a benefit of the reduction in size of the depletedregions 226 a, 226 b, and 226 c is a reduction in size of the p-typediffusion areas 224 a, 224 b, and 224 c, which in turn reduces thejunction area and hence the total area of the parallel plate capacitorformed by the junction. As a result, the photodetector 206 operates as alarge area detector but provides very good (e.g., low) capacitance. Insome embodiments, the decrease in capacitance provided by the decreaseof the total area of the parallel plate capacitor allows for thethickness d₄ of the absorber layer 214 to be decreased as compared withtypical PIN photodetectors. For example, in some embodiments thethickness d₄ of the absorber layer 214 is approximately 3 μm, althoughother thicknesses may be utilized depending on the application. Asdiscussed above, the bulk associated with the absorber layer 214 is afactor in the magnitude of dark currents generated by the photodetector206. Decreasing the thickness d₄ of the absorber layer 214 willtherefore provide a decrease in magnitude of generated dark currents andwill therefore decrease the noise associated with the photodetector 206.

In some embodiments, the anode contacts 220 a, 220 b, and 220 c are thinTitanium (Ti) and Gold (Au) “wires” fabricated on top of thecorresponding p-type diffusion areas 224 a, 224 b, and 224 c. In otherembodiments, the “wires” may be fabricated using various otherconductive materials, including polysilicon and/or Indium:Tin:Oxidematerials. The location of the anode contacts 220 a, 220 b, and 220 creduces the resistance associated with the PIN photodetector 206.Typically, the distance a charge carrier must traverse through thep-type diffusion layer contributes to the resistance provided by thephotodetector. In a typical front-illuminated PIN photodetector, toavoid the anode contact shadowing the detector area, the anode contactsare located to one side, or as a ring around the single, large p-typediffusion area, which increases the resistance associated with the frontilluminated photodiode. In contrast, PIN photodiode 206 shown in FIG. 2locates the anode contacts 220 a, 220 b, and 220 c in the approximatemiddle of the narrow p-type diffusion regions 224 a, 224 b, and 224 c,which decreases the distance traveled across the diffusion region. Inthis way, PIN photodiode 206 provides a relatively low resistance, inparticular as compared with a typical front-illuminated PIN photodiode.In some embodiments, the location of anode contacts 220 a, 220 b, and220 c relative to p-type diffusion areas 224 a, 224 b, and 224 c,respectively, and the relatively short distance charge carriers musttravel across the p-type diffusion areas 224 a, 224 b, and 224 cprovides relatively low photodetector resistance.

In some embodiments, the PIN photodetector 206—utilizing a plurality ofp-type diffusion regions 224 a, 224 b, and 224 c separated from oneanother by a distance d₁—provides low resistance, low dark currentnoise, and low capacitance. As a result, photodetector 206 employed inan optical detection system—such as that shown in FIGS. 1A and1B—provides good overall signal-to-noise ratio. In addition, the lowcapacitance presented by the PIN photodetector 206 provides better noiseperformance and bandwidth performance for the amplifier (e.g.,transimpedance amplifier utilized by signal conditioning circuit 108shown in FIG. 1) utilized to amplify the signals provided by thephotodetector 206. In addition, the PIN photodetector 206 may beutilized as a back illuminated photodetector (light incident onanti-reflective coating 310) or as a front illuminated photodetector(light incident on surface passivationn layer 318).

In some embodiments, a guard ring is provided around the outer perimeterof the plurality of p-type diffusion areas 224 a, 224 b, and 224 c. Insome embodiments, the guard ring includes a guard ring contact 222 and aguard ring p-type diffusion region 225. The guard ring contact 222 isconnected to the voltage source to reverse bias the junctions associatedwith the guard ring, thereby creating a plurality of guard ringdepletion regions 228 surrounding the plurality of depletion regions 226a, 226 b, and 226 c. The purpose of the guard ring is to collectspontaneously generated charge carriers, thereby preventing them frombeing collected by the one or more depletion regions associated withp-type diffusion regions 224 a, 224 b, and 224 c, thereby preventing thespontaneously created charge carriers from contributing to theelectrical response. In some embodiments, the guard ring prevents chargecarriers created far from the depleted regions 226 a, 226 b, and 226 cfrom diffusing to the depleted regions and being collected—therebycontributing to an electrical response that lags behind the peakelectrical response (e.g., slows the response time of the PINphotodetector). In some embodiments, the depletion region 226 a andguard ring depletion region 228 are separated by distance d₅. In someembodiments the distance d₅ is approximately equal to the distance d₃defined between adjacent depletion regions 226 a and 226 b. In otherembodiments, the distance d₅ may be greater than or less than thedistance d₃.

FIG. 3 is a top view of a PIN photodetector 306 according to someembodiments. Visible in the embodiment shown in FIG. 3 is surfacepassivation layer 318, a plurality of anode contacts 320 a-320 i, eachlocated on top of one of a plurality of p-type diffusion regions 324a-324 i, respectively. The n-type substrate, absorber layer, anddepletion regions are not visible. In the embodiment shown in FIG. 3,each of the plurality of anode contacts 320 a-320 i is in electricalcommunication with contact pad 330. In this way, charge carrierscollected by p-type diffusion regions 324 a-324 i are communicated viathe plurality of anode contacts 320 a-320 i to contact pad 330. In turn,contact pad 330 may be connected via a number of well-known means (e.g.,solder bump for flip-chip, wire, tab, etc.) to a chip packaging ordirectly to one or more downstream components (e.g., signal conditioningcircuit). Dashed line is utilized to depict the effective boundary ofthe photodiode optical collection area where optically or spontaneouslygenerated carriers are approximately as likely to arrive to thedownstream components as to the guard ring.

In some embodiments, a guard ring is provided around the outer peripheryof the plurality of p-type diffusion regions 324 a-324 i. The guard ringincludes guard ring contact 322, guard ring p-type diffusion layer 325,and guard ring contact pad 332. In some embodiments, guard ring contactpad 332 may be connected via a number of well-known means (e.g., solderbump, wire, tab, etc.) to a chip packaging or directly to the ground ofthe voltage source utilized to reverse bias the photodiode. In this way,charge carriers collected by the guard ring are not included in theelectrical response provided to downstream signal conditioning circuits.

In the embodiment shown in FIG. 3, in the area located within the dashedline and excluding the contact pad 330, a larger percentage of the areais dedicated to the undiffused n-type InP cap layer 216 than to thep-type diffusion regions 324 a-324 i. As a result, in some embodiments alarger percentage of the absorber layer is undepleted than depleted,meaning that a larger percentage of the absorber layer is dominated bydiffusion transit mechanisms than by drift transit mechanisms. In otherembodiments, the percentage of depleted region and the percentage ofundepleted regions may vary depending on the particular application.

A benefit of the embodiment illustrated in FIG. 3 is that fabrication srelatively straight-forward, wherein p-type diffusion regions arediffused into the n-type cap layer. Long, thin metal contact wires areplaced over the diffused areas to connect the diffused areas to thecontact pad 330. The use of thin metal contact wires 320 a-320 iminimizes optical shadowing for front-illuminated devices.

FIG. 4 is a cross-sectional view of a PIN photodetector 406 according tosome embodiments. The PIN photodetector 406 may be utilized inconjunction with optical detection systems, such as those described withrespect to FIGS. 1A and 1B above. PIN photodetector 406 may be utilizedas a back-illuminated photodetector or front-illuminated photodetector.In some embodiments, the PIN photodetector 406 includes cathode contact408, anti-reflective coating 410, n-type layer 412, absorber layer 414,n-type cap layer 416, surface passivation layer 418, anode contacts 420a, 420 b, and 420 b, guard ring contact 422, p-type diffusion layers 424a, 424 b, and 424 c, guard ring p-type diffusion 425, depleted regions426 a, 426 b, and 426 c, guard ring depleted region 428, conductivebumps 430 a, 430 b, and 430 c, conductive layer 432 and packaging layer434.

As described above with respect to FIG. 2, the PIN photodetector 406shown in FIG. 4 includes a plurality of p-type diffusion regions 424 a,424 b, and 424 c formed within the n-type cap layer 416. As describedabove, the term plurality of p-type regions includes a plurality ofregions separated from one another as a plurality of islands, as well asa plurality of regions connected to one another—for example like aplurality of rows or fingers extending from a base. In each case, theplurality of p-type regions 424 a, 424 b, and 424 c are separated fromone another by a distance d₁. As a result of the distance betweenadjacent p-type diffusion regions, the depleted regions 426 a, 426 b,426 c associated with each p-type diffusion region 424 a, 424 b, and 424c, respectively, are separated by a distance. For example, in theembodiment shown in FIG. 4 the adjacent depletion regions 426 a, 426 b,and 426 c are separated by a distance d₃. In some embodiments, eachdepletion region is separated by the same distance, but this may bevaried in other embodiments. As a result of utilizing a plurality ofp-type diffusion areas 424 a, 424 b, and 424 c, the photodetector 406 ischaracterized by undepleted regions located between adjacent depletedregions (e.g., depleted regions 426 a, 426 b, and 426 c). Electron-holepairs generated within the undepleted regions by an incident photon mustdiffuse into one of the depletion regions 426 a, 426 b, and/or 426 c tobe collected. As discussed above, the time it takes for a charge carrierto diffuse to a depleted region for collection is known as the “transittime”. The transit time depends on one or more factors, such as thespeed at which carriers diffuse through the undepleted regions and thelength of the time the carriers will travel before recombining.Typically, the transit time for a given device is known.

As described above, the distance d₃ between adjacent depletion regions426 a, 426 b, and 426 c is selected such that the transit time of chargecarriers created with the undepleted regions satisfies the bandwidthrequirements of downstream signal processing components (e.g., operatingbandwidth). In some embodiments, the threshold transit time is based onthe rise time of the one or more downstream signal processingcomponents, which may be approximated by the Equation 1, provided above.The distance between adjacent depletion regions 426 a, 426 b, 426 c maybe expressed—for example—as shown in Equations 2 and 3, above, to ensurethat the collected charge carriers contribute to the peak signalprovided to the downstream processing components (e.g., signalconditioning circuit, etc.). In this way, while the charge carriers arenot collected as fast as possible from the photodetector 406 becausethey rely on diffusion mechanisms rather than drift mechanisms, thecharge carriers are collected fast enough for the carriers to beincluded in the downstream signal.

In the embodiment shown in FIG. 3, charge carriers collected by thep-type diffusion regions were communicated via conductive “wires” 320a-320 i to a contact pad 330. In the embodiment shown in FIG. 4, ratherthan utilize a plurality of conductive “wires” fabricated on top of thep-type diffusion layers, the embodiment shown in FIG. 4 electricallyconnects the plurality of p-type diffusion regions 424 a, 424 b, and 424c outside of the PIN photodetector itself. For example, the electricalconnection between the plurality of p-type diffusion regions 424 a, 424b, and 424 c may be made within the packaging or circuitry to which thePIN photodiode 406 is connected. In the embodiment shown in FIG. 4,conductive bumps 430 a, 430 b, and 430 c are in contact with p-typediffusion regions 424 a, 424 b, and 424 c, respectively. The conductivebumps 430 a, 430 b, and 430 c may be solder bumps commonly utilized inflip chip type packaging techniques, or may include other well-knownmeans of connecting a semiconductor device to a package and; or circuit(e.g., flip chip, ball grid array, hybridization, wire bonding, etc.). Aconductive layer 432 provides a conductive path between the plurality ofconductive bumps 430 a, 430 b, and 430 c to provide an electricalresponse representative of charge carriers collected by the plurality ofp-type diffusion regions 424 a, 424 b, and 424 c. In some embodiments,conductive layer 432 is a plurality of conductive “wires” or traces thatprovide an electrical connection between the plurality of p-typediffusion regions 424 a, 424 b, and 424 c. In other embodiments, becausethe plurality of p-type diffusion regions 424 a, 424 b, and 424 c areconnected together, the conductive layer 432 is a conductive sheet. Insome embodiments, packaging layer 434 is an insulative layer thatsupports the conductive layer 432. The conductive layer 432 and/orpackaging layer 434 may be included as part of a chip package, circuitboard, or other off-die device to which the photodetector 406 isconnected. In other embodiments, various other means of collecting andcombining the charge carriers collected by p-type diffusion regions 424a, 424 b, and 424 c may be utilized.

FIG. 5 is a top view of a PIN photodetector according to someembodiments. In the embodiment shown in FIG. 5, PIN photodetector 506includes an array of p-type diffusions 524 a-524 i (i.e., diffusionislands). Each of the p-type diffusions 524 a-524 i is connected to oneanother via a plurality of electrical contacts 520 a-520 i and aplurality of conductive “wires” 536 a-536 d.

In general, FIG. 5 illustrates an array of p-type diffusion islands 524a-524 i that are connected to a single output provided to signalconditioning circuit (e.g., signal conditioning circuit 108 or 108′shown in FIGS. 1A and 1B). Fabrication of a PIN photodetector 506utilizing an array of p-type diffusion islands—and in particular theconductive elements utilized to connect the array of p-type diffusionislands to one another—may utilize several configurations. In someembodiments the connection of the array of p-type diffusion islands 524a-524 i to one another is provided on the semiconductor die itself,while in other embodiments the connection of the array of p-typediffusion islands 524 a-524 i is provided off-die. For example, in someembodiments, the connection of the array of p-type diffusions islands524 a-524 i is provided in the integrated circuit or other type ofpackaging associated with the PIN photodetector 506, such as on theprinted circuit board or ceramic packaging. For example, as describedwith respect to FIG. 4, a plurality of conductive bumps may be utilized,wherein a conductive bump or solder bump is formed on top of each of theplurality of electrical contacts 520 a-520 i, and connected together bya conductive layer. The conductive layer may be formed on an integratedcircuit package, on a printed circuit board, or some other type ofoff-die packaging. In this way, charge carriers collected by theplurality of p-type diffusion regions 524 a-524 i are combined toprovide an electrical response.

As described above, in some embodiments the array of p-type diffusions524 a-524 i are connected to one another on-die. In one embodiment aninsulating layer (not shown) such as an oxide layer is deposited on topof the InP cap layer 518 and array of p-type diffusions 524 a-524 i. Aplurality of vertical vias (extending into the page) are formed overeach of the plurality of p-type diffusions 524 a-524 i, wherein thevertical vias are filled with a conductive material to provide anelectrical path between the plurality of p-type diffusions 524 a-524 iand the vias. In some embodiments, a plurality of wires (such as thoseshown in FIG. 5) are utilized to connect the plurality of conductivevias together and to the contact pad 530. In other embodiments, becauseall of the conductive vias are to be connected to one another, a singleconductive sheet may be placed on top of the insulator to connect theplurality of conductive vias to one another and to the contact pad 530.The guard ring includes guard ring contact 522, guard ring p-typediffusion layer 525, and guard ring contact pad 532.

The present disclosure therefore provides a PIN photodetector that canbe utilized in an optical detection system. The PIN photodetectorutilizes a plurality of p-type diffusion regions. In some embodimentsthe p-type diffusion regions are arranged in a plurality of rows, whilein other embodiments the p-type diffusion regions are arranged as anarray of p-type diffusion islands. The plurality of p-type diffusionsare connected together to provide a single output, which is provided todownstream signal conditioning and processing components. In someembodiments, the distance between the plurality of p-type diffusionregions is related to the operating bandwidth of the downstream signalconditioning and processing components

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A PIN photodetector comprising: an n-typesemiconductor layer; an n-type semiconductor cap layer; a firstplurality of p-type regions located within the n-type semiconductor caplayer and separated from one another by a distance d₁; an absorber layerlocated between the n-type semiconductor layer and the n-typesemiconductor cap layer including the first plurality of p-type regions;and wherein the first plurality of p-type regions are electricallyconnected to one another to provide an electrical response to lightincident to the PIN photodetector.
 2. The PIN photodetector of claim 1,wherein the first plurality of p-type regions are capable of generatinga plurality of depletion regions within the absorber layer separated byundepleted regions within the absorber layer.
 3. The PIN photodetectorof claim 2, wherein the adjacent depletion regions are separated by adistance d₃, wherein the distance d₃ is approximately equal to or lessthan a diffusion length associated with charge carriers in the absorberlayer.
 4. The PIN photodetector of claim 1, further including aplurality of contacts wherein each of the first plurality of p-typeregions includes one of the plurality of contacts.
 5. The PINphotodetector of claim 1, wherein the first plurality of p-type regionsis a plurality of p-type rows diffused within the n-type semiconductorcap layer.
 6. The PIN photodetector of claim 5, wherein the plurality ofp-type rows are connected to one another.
 7. The PIN photodetector ofclaim 5, further including a plurality of thin conductive contact wires,wherein each of the plurality of p-type rows includes a thin conductivecontact wire connected to a contact pad.
 8. The PIN photodetector ofclaim 1, wherein the first plurality of p-type regions is an array ofp-type semiconductor islands diffused within the n-type semiconductorcap layer.
 9. The PIN photodetector of claim 1, wherein the firstplurality of p-type regions are electrically connected to one another onthe PIN photodetector.
 10. The PIN photodetector of claim 1, wherein thefirst plurality of p-type regions are electrically connected to oneanother external to the PIN photodetector.
 11. The PIN photodetector ofclaim 10, wherein the electrical connection of the first plurality ofp-type regions is made within a packaging or carrier associated with thePIN photodetector.
 12. The PIN photodetector of claim 1, furtherincluding a guard ring located on a same plane as the first plurality ofp-type regions, wherein the ring guard surrounds the first plurality ofp-type semiconductor regions.
 13. An optical detection systemcomprising: a PIN photodetector comprising: an n-type semiconductorlayer; an n-type semiconductor cap layer a plurality of p-type diffusionregions diffused within the n-type semiconductor cap layer and separatedfrom one another by a distance d₁; an absorber layer located between then-type semiconductor layer and the n-type semiconductor cap layer,wherein the plurality of p-type diffusion regions are capable ofgenerating a plurality of depletion regions within the absorber layer,wherein each of the depletion regions are separated from adjacentdepletion regions by a distance d₃; and a signal conditioning circuitconnected to receive the electrical response generated by the PINphotodetector, wherein the signal conditioning circuit is characterizedby a bandwidth.
 14. The optical detection system of claim 13, whereinthe distance d₃ between adjacent depletion regions is selected based onthe bandwidth of the signal conditioning circuit.
 15. The opticaldetection system of claim 14, wherein the distance d₃ is selected suchthat a transit time of charge carriers created between the depletionregions is less than 1/bandwidth of the signal conditioning circuit. 16.The optical detection system of claim 13, wherein the distance d₃between adjacent depletion regions is selected based on a rise time ofthe signal conditioning circuit.
 17. The optical detection system ofclaim 16, wherein the distance d₃ is selected such that a transit timeof charge carriers created between the depletion regions is less thanthe rise time of the signal conditioning circuit.
 18. The opticaldetection system of claim 13, wherein the plurality of p-type diffusionregions is a plurality of p-type diffusion rows separated by thedistance d₁.
 19. The optical detection system of claim 13, wherein theplurality of p-type diffusion regions is an array of p-type diffusionislands separated by the distance d₁.
 20. The optical detection systemof claim 13, wherein the PIN photodetector further includes a pluralityof conductive wires in electrical contact with the plurality of p-typediffusion regions to provide an electrical response to charge carrierscollected by the plurality of p-type diffusion regions.
 21. The opticaldetection system of claim 13, wherein an electrical connection of theplurality of p-type diffusion regions is made within a packaging orcarrier associated with the PIN photodetector.